Refractory dissolved organic matter can influence the reproduction of Caenorhabditis elegans (Nematoda)

1. We investigated the effect of refractory dissolved organic matter (refractory DOM: fulvic acids (FAs) and ultrafiltrates (UFs)), isolated from five different sources, on the reproduction of the bacterivorous nematode Caenorhabditis elegans. Nematodes were exposed to DOM (0.5-64 mg L(-1) dissolved organic carbon) during a whole life cycle (72 h). At the end of the test, the number of offspring per worm was determined. 2. We also studied the effect of refractory DOM on abundance, cell size, and activity of the bacteria (Escherichia coli) that were used as a food source for the nematodes, to assess possible indirect effects of DOM via the food organisms. 3. The effects of DOM on the reproduction of C. elegans varied, depending on the origin and concentration of DOM. FAs isolated from a soil leachate and from the effluent of a waste water plant, as well as UFs from a humic lake and from a marsh, stimulated the reproduction of C. elegans. FAs from ground water had no effect, while FAs from a humic lake inhibited the reproduction of the nematodes. All effects occurred at ecologically relevant DOM concentrations and showed clear dose-response relationships. 4. Neither bacterial abundance nor cell size were influenced by refractory DOM. Bacterial activity was unaffected by four types of DOM. Only FAs from the humic lake caused a significant decrease in bacterial activity over 72 h. 5. The negative effect of FAs from the humic lake on nematode reproduction may be a consequence of a lower bacterial activity in this treatment. The positive effects of refractory DOM, however, could not be related to bacterial parameters. Therefore, we assume that the DOM directly influenced the reproduction of C. elegans. We speculate that refractory DOM can potentially be an additional carbon source or a source of trace nutrients influencing the reproduction of C. elegans. Adsorption of refractory DOM on bacterial cells, serving as food for the nematodes, may have been an important factor for the availability of DOM for C. elegans.